Plants possess tolerance mechanisms to cope with various types of environmental stresses in nature, such as dehydration, high temperature, freezing, or salt stress. As the stress tolerance mechanism has been elucidated at a molecular level in recent years, stress tolerant plants have been produced via biotechnological techniques. For example, it has been shown that stress proteins such as LEA proteins, water channel proteins, or synthases for compatible solutes are induced in cells when they are exposed to stress, thereby protecting the cells from such stress. Thus, research has been attempted in which genes of LEA proteins of barley or detoxification enzymes of tobacco, genes of synthases for osmoregulatory substances (e.g., sugar, proline, or glycinebetaine), or the like are introduced into host plants. Research using genes encoding w-3 fatty acid desaturase of Arabidopsis thaliana, the D9-desaturase of blue-green algae, or the like, which are modification enzymes of the cellular membrane lipid, has also been attempted. In such research, a gene was bound to the 35S promoter of the cauliflower mosaic virus and introduced into a plant. The level of stress tolerance of the recombinant plant was, however, unstable, and the expression level of the introduced gene was low. Thus, none of these was put to practical use.
On the other hand, a stress tolerance mechanism is found to be intricately associated with several genes (Shinozaki K, Yamaguchi-Shinozaki K. Plant Physiol., 1997, October; 115(2), pp. 327-334). Accordingly, research whereby a gene that encodes a transcription factor and that also simultaneously activates the expression of the aforementioned several genes is ligated to a constitutive promoter and introduced into a plant, thereby enhancing the plant's stress tolerance, has been attempted (Liu et al., The Plant Cell, 1998, 10: 1391-1406). When expressions of several genes are simultaneously activated, however, the energy of the host plant becomes directed towards the synthesis of the gene product or intracellular metabolism resulting from the gene product. Accordingly, the growth of the plant itself becomes retarded or results in a dwarf.
In contrast, the present inventors isolated from Arabidopsis thaliana the DREB1A, DREB1B, DREB1C, DREB2A, and DREB2B genes encoding the transcription factors that bind to a stress-responsive element and specifically activate the transcription of genes located downstream of such element (JP Patent Publication (Unexamined Application) No. 2000-60558). They reported that the introduction of the genes into a plant by ligating them to a stress-inducible rd29A promoter enabled production of a stress-tolerant plant without retarding plant growth (JP Patent Publication (Unexamined Application) No. 2000-116260).
The rd29A promoter is derived from Arabidopsis thaliana, which is a dicotyledonous plant. It is able to function in monocotyledonous plants, although its activity level is low. Accordingly, a stress-inducible promoter capable of a high level of activity in monocotyledonous plants has been awaited.